Conventionally, as shown in FIG. 1, there is known a heavy load tire that has, in its tread portion 10, a protection belt ply 11 composed of two sheets of protection belts 11A/11B, a main crossed belt ply 12 composed of two sheets of main crossed belts 12A/12B, and a small crossed belt ply 13 composed of two sheets of small crossed belts 13A/13B (for example, see Patent Documents 1 and 2).
As shown in FIG. 1, in the tire 1, the main crossed belt ply 12 is disposed on an outer side of the small crossed belt ply 13 in a tire radial direction, and the protection belt ply 11 is disposed on an outer side of the main crossed belt ply 12 in the tire radial direction.
For example, in the tire 1, an angle between cords constructing the small crossed belt ply 13 and a tire circumferential direction L is 4-10°, an angle between cords constructing the main crossed belt ply 12 and the tire circumferential direction L is 18-35°, and an angle between cords constructing the protection belt ply 11 and the tire circumferential direction L is 22-33°.
Therefore, in the tread portion 10 of the tire 1, an area near a tire equator line CL (a central area) has smaller angles formed between cords constructing the belt ply and the tire circumferential direction L than an area near an end portion in a tire width direction W (a shoulder area).
In the above-explained tire 1, a belt tension becomes small in the area having a large angle between cords constructing the belt ply and the tire circumferential direction L, so that the area contracts greatly along the tire circumferential direction L.
As a result, when the tire 1 rotates, the area near the end portion in the tire width direction W contracts greatly along the tire circumferential direction L, so that a length, along the tire circumferential direction L, of the area near the tire equator line CL becomes longer than a length, along the tire circumferential direction L, of the area near the end portion in the tire width direction W.
Therefore, when the tire 1 rotates, a force in a tire rotational direction (driving force) is generated in the area near the tire equator line CL and a force in an opposite direction to the tire rotational direction (braking force) is generated in the area near the end portion in the tire width direction W, so that a shearing force is generated near the boundary of the both areas.
Further, in a case where a load is applied to the tire 1 after an inner pressure is added to it, a shearing force is generated near the boundary of the both areas, because degrees of deformations along the tire radial direction are different between the area near the tire equator line CL and the area near the end portion in the tire width direction W.
Especially, a force in the tire width direction W is added due to a steered angle in a case where the tire 1 is installed onto a steered axle, and the shearing force is made much larger by the applied breaking force in a case where the tire is installed onto an axle to which a breaking force is applied.
The above phenomena are particularly accentuated in the heavy load tire 1 that is constituted so that a length of a land portion(s) along the tire width direction W is not smaller than 30% of a length of the tread portion 10 along the tire width direction W.